As described by Norling in U.S. Pat. No. 5,367,217, FOUR BAR RESONATING FORCE TRANSDUCER,” the complete disclosure of which is incorporated herein by reference, two- and four-bar vibrating beam force sensing elements have been used as vibratory resonators in crystal-controlled oscillators and are generally known in the art. Such force sensing elements have been known to be used in various transducers to measure various parameters, including acceleration, force, temperature, pressure and weight. In particular, such vibrating beam force sensing elements are responsive to forces, such as longitudinal or axial forces, which cause a variation of the frequency of vibration of the beams that, in turn, cause a variation in an output frequency of the oscillator which can be used as a measure of the applied force.
In U.S. Pat. No. 5,501,103, “TWO-PORT ELECTROMAGNETIC DRIVE FOR A DOUBLE-ENDED TUNING FORK,” the complete disclosure of which is incorporated herein by reference, Woodruff, et al. describes a vibrating beam accelerometer which includes two- and four-bar electromagnetically excited double-ended tuning forks (DETF) of the type described by Norling. As described by Woodruff, et al., vibrating beam accelerometers are generally known in the art. The DETF of Woodruff is formed with separate conducting paths for the drive circuit and pick-off circuit that minimize problems associated with vibrating beam accelerometers formed with a single conducting path resulting from variations of the resistance path due to manufacturing tolerances and temperature changes.
FIG. 1 illustrates one example of a known vibrating beam accelerometer 1. Such known vibrating beam accelerometers normally include a pendulum or proof mass 3, connected to a frame 5 by way of a pair of flexures 7 to enable the proof mass 3 to rotate about a hinge axis HA, defined by the flexures 7. A double-ended tuning fork (DETF) having two or four vibrating beams is connected between the frame 5 and the proof mass 3, perpendicular to the hinge axis HA to define a sensitive axis SA. In the example of FIG. 1, a DETF 9 having four vibrating beams 11, 13, 15 and 17 is connected between the frame 5 and the proof mass 3. Excitation is applied to the DETF 9 to cause the vibrating beams 11, 13, 15 and 17 to vibrate at a resonant frequency when the proof mass 3 is at rest. Forces applied along the sensitive axis SA apply either tension or compression forces the vibrating beams 11, 13, 15 and 17 which changes their resonant frequency. This change in frequency, in turn, is a measure of the force applied along the sensitive axis SA.
Various types of excitation are known to force the vibrating beams 11, 13, 15 and 17 to vibrate, such as electromagnetic, electrostatic, and thermal excitation. The type of excitation depends on the particular materials used for construction. For example, crystalline quartz DETFs are excited according to the piezoelectric property of the quartz. Silicon DETFs are normally micromachined and are excited by other means, such as electrostatically or electromagnetically.
FIG. 1 illustrates an exemplary silicon micromachined vibrating beam accelerometer that includes a four-beam double-ended tuning fork that is adapted to be excited electromagnetically. In such an embodiment, the outer pair of vibrating beams 11 and 17 are electrically connected together by a conducting member 19. The inner pair of vibrating beams 13 and 15 is connected together by a conducting member 21. Free ends of each of the vibrating beams 11, 13, 15 and 17 are connected to wire bond pads 23, 25, 27 and 29. In such an embodiment, a conductive material, such as gold, is applied to the vibrating beams 11, 13, 15 and 17 as well as the wire bond pads 23, 25, 27 and 29 to enable electric current to flow between the wire bond pads 23, 25, 27 and 29 through the respective vibrating beams 11, 13, 15 and 17. Such a configuration provides separate conducting paths between the vibrating beams used for the drive circuit and the vibrating beams used for the pick-off voltage. In particular, a first conducting path is formed between the outer pair of beams 11 and 17, while a second conducting path is provided between the inner pair of tines 13 and 15.
An oscillator circuit is provided to drive the vibrating beams. In particular, the outer pair of vibrating beams 11, 17 are used in the drive circuit, while the inner pair of vibrating beams 13 and 15 are used in the pick-off circuit. Referring first to the drive circuit, the electrode 29 is connected to ground by way of an electrical conductor 31. The other inner vibrating beam 13 is connected to an amplifier 33 by way of an electrical conductor 35 connected to the electrode 25. The output of the amplifier 33 is, in turn, connected to an amplitude limiter 37 by way of an electrical conductor 39. The output of the amplitude limiter 37, in turn, is used to provide an alternating current (AC) drive current of the outer pair of vibrating beams 11, 17. In particular, the output of the amplitude limiter 37 is connected to the electrode 27 by way of an electrical conductor 41. This forces the drive current up the outer beam 17 and down the outer beam 11 to ground. The drive current is then connected to ground by way of the electrical conductor 43. An externally generated magnetic field B is applied in a direction generally perpendicular to the plane of the DETF 9. The magnetic field B having flux lines in a direction generally perpendicular to the plane of the DETF, interacts with the AC drive current in the outer beams 11 and 17 which causes these beams 11, 17 to vibrate. Mechanical couplings between the pair of beams 11 and 13 and between the pair of beams 15 and 17 are provided by respective cross members 45 and 47. More particularly, one cross member 45 is connected between the tine 11 and 13 to cause these beams to vibrate together. The other cross member 47 is connected between the beams 15 and 17 to cause them to vibrate together. Since the cross member 45 is connected between the beams 11 and 13 and the cross member 47 is connected between the beams 15 and 17, all four beams 11, 13, 15 and 17 are mechanically coupled together forming a two degree of freedom mechanical system. These mechanical couplings of the inner pair of beams 13 and 15 relative to the outer pair of beams 11 and 17 cause a voltage to be generated across the inner pair of beams 13 and 15. This voltage is generated across the electrodes 25 and 29. This voltage, known as the pickoff voltage, is then applied to an amplifier 33 by way of a positive feedback loop in order to form an oscillator.
In operation, in response to an excitation or drive current the beams 11, 13, 15 and 17 are forced to vibrate at a resonant frequency while the proof mass 3 is at rest. Force applied to the proof mass 3 along the sensitive axis SA causes the vibrating beams 11, 13, 15 and 17 to undergo either tension or compression which, in turn, causes a variation in the resonant frequency at which the beams 11, 13, 15 and 17 vibrate. This variation in the resonant frequency is useful a measure of the applied force. This frequency can be measured at the output of the amplifier 33 along a signal line 49 by any conventional frequency measuring circuitry which is well known in the art. According to known prior art, the vibrating beam accelerometer device 1 optionally includes a second DETF sensor 9′ coupled between the an end of the proof mass 3 and frame 5 opposite from the suspension flexures 7. Such a dual DETF arrangement provides many advantages, such as doubling the output and common mode cancellation of error sources, which is the tracking and mutual cancellation of the common mode responses of two DETFs in a single sensor.
Vibrating beam accelerometer of the type depicted in FIG. 1 have been fabricated from a body of semiconductor material, such as silicon, using micromachining techniques as microelectromechanical systems, or “MEMS,” integrated micro devices or systems combining electrical and mechanical components fabricated using integrated circuit (IC) batch processing techniques.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting free-standing three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, “Method of Making an Electrostatic Silicon Accelerometer” and U.S. Pat. No. 4,945,765 “SILICON MICROMACHINED ACCELEROMETER,” the complete disclosures of which are incorporated herein by reference.
Vibrating beam accelerometer of the type depicted in FIG. 1 have different features provided in the front and back surfaces 50, 51. For example, features of the DETF 9 are provided in the front surface 50 while features of the suspension flexures 7 are formed in the back surface 51. Such two-sided structures have been formed using different techniques with different results. For example, a large array of the moving system (proof mass 3, frame 5, suspension flexures 7, etc.) is fabricated in one wafer substrate while another large array of the DETF sensors 9 is fabricated in a different wafer substrate. The DETF sensors 9 are then attached between proof mass and frame, as by an adhesive or other bonding agent. A vibrating beam accelerometer having such an adhesively bonded DETF sensor is described by Woodruff, et al. in U.S. Pat. No. 6,484,578, “VIBRATING BEAM ACCELEROMETER.” While effective for some applications, this bonding of the DETF sensor 9 introduces an area of inherent thermal mismatch that leads to inaccuracies in the sensor output that is unacceptable in high accuracy applications. Furthermore, because the features, particularly the DETF sensors 9, are so small and delicate, adhesive or other bonding of the DETF sensors 9 is not known to be feasible using today's assembly techniques. Rather, the method of forming two-sided structures by adhesive or other bonding of the DETF sensors 9 is currently feasible only in quartz because the structures are larger and stronger than those formed in silicon, and are therefore more easily manipulated.
One effective alternative is fabricating a large array of features in a first side, such as the features of the DETF 9 in the front side 50, of a silicon wafer, then masking these front side features to protect them from further etching, and only then fabricating a matching array of the backside features, such as the suspension flexures 7, in the second side of the wafer. This process of etching the entire array of devices from both sides of the wafer is effective for providing all the device's features integrally in a single substrate without introducing adhesives other bonding agents so that a highly accurate device results. One drawback to such double-sided fabrication is yield which requires high yields of both the first- and second-side features. For example, even masking the first side cannot prevent some damage of the first-side features so that overall yield suffers even more, regardless of the yield of second-side features. Additionally, high yields of the first-side processing requires high yields of the second-side processing as well, else the first-side processing is wasted. In practice, even if the yield of the first-side processing is 70 to 90 percent, if yield of the second-side processing is only 10 percent, the yield for the entire batch is no more than 10 percent. Obviously, a low yield of the first-side results in scrap of the entire batch and second side processing does not occur.
Therefore, a more reliable double-sided fabrication process is desirable.